Variable electrical filter



April 14, 1953 G. L. GRlsDALE 2,635,143

VARIABLE ELECTRICAL FILTER April 14, 1953 G. L. GRISDALE 2,635,143

Y VARIABLE ELECTRICAL FILTER i Filed Jan. 4, 1949 2 SHEETS-SHEET 2 4016/5 m W /V/ ran/5 w Patented Apr. 14, ,1953

.ApplicatiomJanua-ryA, 1945i,v SerialN 0.169,146 .'Inreatfritain February', 19.48

j This .invention .relates vtoi variable .electrical nlters and ,hasiorits .object topprovideizimproved.. and `Asimple'filters of. variablehand ,pass characf teristics. suitable for .use in` radio,.;hi`gh.. speed telegraphy and for similarpurposes.

AccordingV tothis inventioninits l.broadest aspect.. a variable, 'band v'pass .lterarrangement comprises .a rst ..1ilter, a .second filter, and.` The-.- tween said'. filters, .a frecuencychanger havingl a variable" local... oscillation frequency, tthe.- viz-hole arrangement ...beingv such. that, Y :by .var-ying.. said.

local foscillation frequency, the pass..b.and. ofy .one filter.. may, l effect. .be` slid atoavariable. .extent across .the. pass` bamdxof .the-.otherso that-:the wholeerrangement ,constitutes abandpassflter whose pass -widthf may be: adjusted by t.adjusting the local oscillationfrequency.y

important .advantage v-of `the `invention-.lies iii-fthe fact ethat .the separateeilters ,needA nothe designed-:to 1have/sharp .cut-off on bothfsides K.01?l

their pass .bands vbut .only on one lside ;and,;in

many .cases they mayfbe :merely .-hieh-passgorlow Dassflters; I 1

.Preferablythenrstfniter.is pneeededibyefseeond 'Y frequency changerand: the isecondzltereis -zfoblowectby. a'thi-rdtreqnency changerfbothlwliich have Avri-riablelocal:oscillation frequencies-fyvhch. aressocrelatedi-to tha-tof .theflrstioscillationifr Y quency: :that f -thefmid-'irequency .oi-.their variable.

widthtband :.fromf-the; seconfdzfrequencyfehameer: nemainsyconstanta. as the widthziof bandpis; varied.-` Infsuchv-.aacase the.threeioscillationifref quenqies :may-"oe generated :by threesclators: whichiaregangontrolledr they maygbe .derined` fromY means. --includinggra common i'freqnency;

source;

1Ti-ie.two;.f1ters-.abovemeierredtmand employed: for. #variable4 band width lriceiliition':may.Ai-,also1fbe usedfortheseparationnfsidebandsiinsingleside@l band-systems: and, (by: providing Suitable :switch-.-A ing,; -variable selectivity .and .-.sidelcandsepcitation-v maybeobtainedimoneeguipment.

Y.The invention is.illustratedinthe/accompany!-' ingedrawings which .illustrate :agnumberfoiziemfbodiments .umich .will.be.described.withiexamplea It, :is to .be uindensteod;V however, .that `these. frequencies `arefgivcli bymay. ofexampleonly andltoassistin .clear .understandin of. factual. ,frequencies` ing. .the yinvention being in no .Way limited .to these examples.

Figs. ..1 .to..3 .inclusive ereifblockfdiagrams fill-ustrating variable `mnd-pass..iilters `according .t0- iny, invention..

Bies 4 .and .-.a1te..b1ock..diagrams oli-receivers embodying the invention.

an upper, side .band extending ...110.2303 -zfkcf bodiment of -the-inverrtionas-applied Vtoprovide variable Y'rba-nd-fpassselectivity' rin vthe vintermediv ate-frequency '(Y'I; =l) stages of a superheterodyneample, in thejpresenttoase they'may bevdesigned to have Asharp -cut-ojif' at'the uppen'frequeney limitsonly.

yIt will Lbe' :seem that, Yin 'the above embodiment Whenu the localoscillator :LO- isc adjustedgtopass kthe second "lter",B.- `When',V however, the. local oscillators 'adjusted' to "269. keg/s; 'the band this 'only tlxeportion '15.0-514'9 (correspondito thewfirst'jlter-)cWill-pass theiseeond iilter. .Inj

this way-the local'oscillatonprovides van adjustment-ofthe overall pa'ss hand Widthzo 'irom I0-kc./s:to-1"kc. /.s. g

when'the-fband Widthis reduced to1.kc.s.1. .This may, in Imany Acases,.zLbe objectionable; ier. ex.

.ampleit wouldcauseythe 'beat note .of .,a C. W. receiverto valiyand would Ieadtoidifficulties With` automaticfregueney .cntraol LCAaEdCl) .inthe-case I of a. receiverhaving and.A.l-'1.'iCLfsystemonerated 'by-fthe output ,from .thesecondlter-..

VThis defect can'be avoided "by providingaddi--l tional V.frequency .changerswith variablelocaloscllation'. .ireguenciesin cascade onefeach Eszdezfof" an tarrangementas above ,described .Such4 an improved .arrangement is illustrated in'..Figure. ,2 as appliedtofthe oase 0fa,tr1`,p1e 4detectionlsuperheterodynegreceiver having, vin a Ciirst.(inter-nicely ate .frequency circuit, a.carrier-of.-23 00.kc.s.with

9.j1owersideband .enexil-iing....to` 22er: .kc/slm Figure 2, '1n which the present invention is apndi.

plied to this case to provide a high and variable selectivity, the band 2297-2303 kc./s. is applied to a mixer Ml with which is associated a local oscillator OI which is variable over the range 24253-24205 kc./s. Following the mixer MI is a band pass filter Fl for the range 120-.126 kc./s. and having a sharp cut off at the lowerlimitonly. This is in turn followed by a mixer M2 with which is associated a local oscillator O2 variable over e the range 266-261 kc./s. The mixer M2 feeds into a band pass lter F2 for the range 140-146 kc./s. again having a sharp cut-off at the lower frequency limit only. Output from this filter is taken to a mixer M3 with which is associated a local oscillator O3 variable over the range 24S-240.5 kc./s. The three local oscillations are co-related in their Variation in such'manner as 100-99.5`kc./S. and 1D0-100.5 lic/S.

vThe three local oscillations can, theoretically,

be derived from three gang-controlled oscillatorsv but it is practically much better to derive them from means including a common source. In one example of this nature suitable for use in the embodiment just described and illustrated in Figure 3 there are employed four oscillators OA, OB, OC, OD respectively in association with mixers and band pass filters. Oscillator OA is of 500 kc./s.i4 kc./s.; oscillator OB is crystal controlled at 1680 kc./s.; oscillator OC is crystal controlled at 220 kc./s.; and oscillator OD is variable over the range 23-20.5 kc./s. A filter FE selects the second harmonic range 46-41 kc./s. from oscillator OD and applies it to a mixer MF to which is also applied the 220 kc./s. from oscillator OC. A filter FG selects fromA the output from MF the band 26S-261 kc./s. to constitute the variable local oscillation which is applied to the mixer M2 in Figure 2 and is shown in that figure as derived from an oscillator O2 The output from oscillator ODl is also fed to a mixer MH which also receives output from oscillator OC and a filter FJ selects, from the output from MH the band 24S-240.5 to constitute the variable local oscillation frequency for the mixer M3 in Figure 2 and shown in that figure as derived from oscillator'OS.' Outputs from oscillators OA and OB are combined in a mixer-'selector MK which selects the frequency of 2180 kc./s. and feeds it to a mixer ML which also receives the band of 24S-240.5 lio/s. from filter FJ. A filter FM selects from the output from mixer ML the band 2423-24305 kc./s. required as the variable local oscillation frequency for the mixer` MI in Figure 2 and shown in that figure as' derived from Ol. I Another advantage of the invention lies in the fact that it lends itself to the ready construction of a receiver which, by a simple switching change, can be readily changed over from high, variable selectivity double side band reception on one channel to single side band double channel reception. Thus, for example, by changing the connections of the elements contained in the high Variable selectivity double` side band single channel arrangement illustrated in Figure 2 and adding a comparatively few elements as represented in Figure 4 the said arrangement can be changed over to double channel single side band reception, elements provided for use in single channel reception being thus used for double channel reception with the addition of further mixers and local oscillators. The switching changes for changing over to double channel reception are such as to produce the circuit of Figure 4 in which there are three parrallel paths, one for each side band and one for the carrier, the carrier and the two side bands-namely 2300 kc./s., 2300-2306 kc./s. and Z300-2294 kc./s.-being applied to all three paths in parallel. Such switching arrangement provides circuit connections in the upper side band path through the mixer MI, with which is associated the oscillator Ol (adjusted to 2420 kc./s.) that is followed by the filter Fi (120-126 kc./s.) which is in turn followed by the rst mixer M2 with which is associated the oscillator O2 (adjusted to 220 kc./s.) The output from M2 comprises the lower side band 94-100 kc./s. and is taken to one demodulator DI to which is also applied carrier (at l00kc./s.) from the carrier 'the carrier path. The carrier path consists simply of a mixer M5 with which is associated an oscillator O6 at 2200 kc./s. and which may be followed by an amplifying carrier iilter AMF at 100 kc./s."Carrier output from the ampliier AMF may be used for automatic frequency control as indicated by the reference AFC. The same elements are indicated by the same references in Figures 2 and 4 since the latter figure is' a re-connection of the elements of the former figure 'with further elements added.

The oscillator O6 of the carrier path, and the oscillators MI, Mil of the two side band paths should be constituted by different output circuits of 'a multiple oscillator unit on the lines of that already described as preferred for single channel reception. A convenient arrangement is shown in Figure 5 and comprises four oscillators O1, O8, O5 and O2 of, respectively, 500 kc./s.i4 kc./s.; 2700 kc./s. (crystal controlled); 40 kc./s. (crystal controlled) and 220 kc./s. (crystal controlled). The oscillators Oi, O8 (of which the former is subjected to automatic frequency control as indicated by the legend AFC) feed into a mixer-selector Ml' which selects 2200 kc./s. for use as the carrier path local oscillation for feeding to mixer M6 (Figure 4). The output from mixer Mi is fed to two other mixers M6, M9 of which M8 also receives the 40v kc./s. oscillation from O5 to produce the required 2160 kc./s oscillation for the mixer M4 (Figure 4) in the lower side band path, and the other M9 also receives the 220 kc./s. from O2 to produce the 2420 kc./s. oscillation required for the mixer Ml (Figure 4) in the upper'side band path. The two remaining oscillations of 40 kc./s. (for M5 in- Figure 4) and 220 kcL/s. (for M2 in Figure 4) are derived direct from the oscillators O5 and O2 respectively. The same elements are indicated by the same references in Figures 4 and 5.

I claim:

1. A variable band pass filter arrangement comprising in combination a first band pass filter of a predetermined band width and having a sharp cut off on at least one side of its pass band, a second band pass filter of the same order of band Width and having a sharp cut off on at least one side of its pass band, a frequency changer, means for applying the output from the first filter as one of the inputs to said frequency changer, a local oscillator variable over a predetermined frequency range of the same order as the pass bands of said filters, means for applying oscillations from said local oscillator as the second input to said frequency changer, and means for applying the output from said frequency changer as input to said second filter, said predetermined frequency range being so chosen in relation to said pass bands that the resultant frequency band derived from the combinations of the frequencies of the pass band of said first filter with one end frequency of said frequency range overlaps the pass band of said second filter by an amount different from that by which the resultant frequency band derived from the combinations of the frequencies of the pass band of said first filter with the other end frequency of said frequency range overlaps the pass band of said second filter, a second frequency changer preceding the first filter and connected to apply its output as input thereto, a second variable frequency local oscillator feeding into said second frequency changer, a third frequency changer fed by the second filter, a third variable frequency local oscillator feeding into said third frequency changer and means co-relating the frequencies from the three local oscillators to hold them during variation of their frequencies, in predetermined relation to maintain the mid-frequency of the variable band from the second frequency changer substantially constant during variation whereby the width of the band passed by the whole arrangement may be varied by varying said oscillator frequency over said frequency range.

2. A variable band pass filter arrangement comprising in combination a first band pass filter of a predetermined band width and having a sharp cut off on at least one side of its pass band, a second band pass filter of the same order of band width and having a sharp cut off on at least one side of its pass band, a frequency changer, means for applying the output from the first filter as one of the inputs to said frequency changer, a, local oscillator variable over a predetermined frequency range of the same order as the pass bands of said filters, means for apply oscillations from said local oscillator as the second input to said frequency changer, and means for applying the output from said frequency changer as input to said second filter, said predetermined frequency range being so chosen in relation to said pass bands that the resultant frequency band derived from the combinations of the frequencies of the pass band of said first filter with one end frequency of said frequency range overlaps the pass band of said second filter by an amount different from that by which the resultant frequency band derived from the combinations of the frequencies of the pass band of said first filter with the other end frequency of said frequency range overlaps the pass band of said second filter, a second frequency changer preceding the first filter and connected to apply its output as input thereto, a second variable frequency local oscillator feeding into said second frequency changer, a third frequency changer fed by the Second filter, a third variable frequency local oscillator feeding into said third frequency changer and gang control means for the three local oscillators to hold them during variation of their frequencies in predetermined relation to maintain the mid-frequency of the variable band from the second frequency changer substantially constant during variation whereby the width of the band passed by the whole arrangement may be varied by varying said oscillator frequency over said frequency range.

GEORGE LAMBERT GRISDALE.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,830,896 Wintrington Nov. l0, 1931 2,282,974 Koch May 12, 1942 2,354,749 Griffin Aug. 1, 1944 2,364,863 McLaughlin Dec. l2, 1944 2,472,798 Fredendall June 14, 1949 

